The practical use of optical communication systems has been promoted these days. In this situation, highly advanced optical communication systems having, for instance, large capacity, multi-functions, etc. have been sought. In this tendency, the generation of light signals at a high speed, and the highly developed change-over and switching of light signals have been required to be proposed for a practical use.
In a conventional optical communication system, light signals are obtained by direct-modulating current injected into a semiconductor laser or a light emitting diode. However, this direct modulation is difficult to be realized at a speed higher than approximately 10 GHz due to the effect of relaxation oscillation, and to be applied to a coherent light transmission system due to the occurrence of wavelength fluctuation. For the purpose of overcoming these disadvantages, an external modulator has been used in an optical communication system. Especially, an optical modulator of a waveguide type having waveguides including a directional coupler provided on a substrate is advantageous in that it is of a compact, a high efficiency, and a high speed.
On the other hand, an optical switch has been used to change-over light transmission paths and to carry out the switching of light signals in a network system. A conventional optical switch is of a structure having a prism, a mirror, an optical fiber, etc. which are adapted to move mechanically. For this structure, the conventional optical switch is of a low speed, a low reliability, a large size, etc. so that it is difficult to be applied to an optical circuit of a matrix pattern. An optical switch which has been developed to overcome these disadvantages is also of a type having optical waveguides including a directional coupler to realize high speed operation, the integration of devices, high reliable operation, etc. Especially, an optical switch utilizing a ferroelectric material such as lithium niobate (LiNbO.sub.3) crystal, etc. is advantageous in that light absorption is low to provide a low loss, and electro-optic effect is high to provide a high efficiency. As such an optical control device, a directional coupler type optical modulator/switch, a total reflection type optical switch, Mach-Zehnder type optical modulator, etc. have been conventionally reported. When this waveguide type optical control device is applied to a practically operated optical communication system, it is indispensable to provide the reproducibility of operative characteristics deeply connected to a high yield of devices along with basic performances such as low loss, high speed, etc.
On type of a conventional optical control device has first and second optical waveguides provided on a substrate of lithium niobate. The optical waveguides are partly narrowered in parallel interval to provide a directional coupler on the substrate. The directional coupler is covered with a buffer layer, on which first and second control electrodes are provided correspondingly to the first and second optical waveguides composing the directional coupler. Both ends of the first and second optical waveguides provide first and second light signal input terminals, and first and second light signal output terminals.
In operation, a light signal supplied to the first light signal input terminal is propagated through the first optical waveguide to be supplied to the directional coupler. When no voltage is applied across the first and second control electrodes, the light signal is completely coupled in the directional coupler to be transferred from the first optical waveguide to the second optical waveguide. Then, the light signal is propagated through the second optical waveguide to be supplied from the second light signal output terminal to a following stage. On the other hand, when a predetermined voltage is applied across the first and second control electrodes, the coupling between the first and second optical waveguides becomes lowered in the directional coupler. The details of the conventional optical control device will be explained in more detail later.
However, the conventional optical control device has a disadvantage in that the characteristics of the optical waveguides changes to change a coupling state of the directional coupler, because the optical waveguides are affected in the vicinity of the control electrodes by the fluctuation of a refractive index which occurs in the substrate of the ferroelectric crystal in accordance with the piezoelectricity and the optical elastic effect by distortion locally accumulated in the vicinity of the control electrodes. In fabricating the conventional optical control device, a control electrode film is grown on the buffer layer, and the control electrode film is etched to provide the control electrodes by use of a mask having a predetermined pattern. The control electrodes provides elastic discontinuity on the substrate, so that the local distortion occurs in the vicinity of the control electrodes on the substrate.